When I installed a coroplast underlining on my Springdale, I noticed that there was little support between the frame rails. I installed carpet edging trim from rail to rail across the bottom about every 4 feet. This gave the coroplast added support and I was able to position it where I could screw it into the frame crossmembers. It held for the 3+ years that we had that trailer and stopped the sagging from side to side.
Here's one link to what I used:
http://www.lowes.com/pd_16297-81-782...rim&facetInfo=